Dissolve the sugar in 4 to 5 tablespoons of water and simmer to get a syrup of 1 string consistency.
Add the walnuts and cashewnuts and mix well. Heat over a slow flame and stir till it leaves the sides of the pan.
Remove from the fire, transfer to another plate and cool completely.
Divide the mixture in 10 equal portions. Press each portion into 25 mm. (1") diameter greased tart moulds.
Unmould the tarts and keep aside.

For the topping

Heat the cream in a pan and bring it to a boil. Add the chocolate and mix well to get a smooth sauce, making sure that no lumps remain.
Cool completely. Pour into a piping bag fitted with a star nozzle and keep aside.

How to proceed

Pipe a swirl of the topping into each walnut tart, using a piping bag.
Garnish with the walnut pieces.